How to add values from textfield form submition into serialized array?

Hi, iam trying to create a wordpress jquery slideshow plugin. I have inserted a serialized array values into wp_option table by using below code.

$options = array(
		'width=> '240px',
		'height' => '240px ',
'img_id' => array(1)
		);
	add_option('myopp',$options);

the above code added the below output .
a:3:{s:5:"width";s:5:"240px";s:6:"height";s:5:"240px";s:6:"img_id";a:1:{i:0;i:1;}}
What I want is, I want to add img_id like 1,2,3 into img_id array key whenever I submit a value through textbox from my form. So how can I do that?
Please help anybody.